1. FIELD OF THE INVENTION
The present invention relates to a method for making clad metal sheets each having a curved facing or cladding metal layer by explosive welding and clad metal sheets produced by this method.
2. DESCRIPTION OF THE PRIOR ART
Explosive welding has been widely used for bonding foreign metals and is very effective especially when both the surfaces to be bonded are flat. Therefore explosive welding has been advantageously used in making clad metal sheets consisting of various combinations of foreign metals. In the process, the surfaces to be bonded of a base and cladding metals are cleaned to exhibit pure metallic surfaces, and the base and cladding metals are spaced apart from each other by a suitable distance when explosive-welded with an explosive whose quantity is selected depending upon the thickness of the base and cladding metals and the space therebetween.
In addition to the welding between sheets, the explosive welding has been recently used for joining between tubes and between tubes and rods. Since the bond provided by the explosive welding is very satisfactory, it is now used widely in various fields.
As the excellent properties and economical advantages of clad metals are more and more recognized in various technical fields, there arises a demand for adding a cladding metal sheet to a curved surface of a base metal. This demand is especially strong in the field of the production of molds for continous casting.
In general the main body of a mold for continuous casting is made of copper having an excellent thermal conductivity, and in order to prevent the abrasion and wear of the inner surfaces of the mold and to prevent the adhesion of copper to a billet being cast, the inner surfaces are in general coated with a foreign metal such as nickel, chromium or the like by the so-called metal plating method. In some molds, clad metal sheets made by adding a foreign metal only to the upper section by the explosive welding are used in order to protect the inner surfaces of the mold subjected to severe abrasion and wear.
However, the conventional plating methods may provide a facing or lining very thin in thickness so that the lifetime is very short. Since the molten steel is gradually solidified as it flows down in the mold, the thickness of the facing or metal lining must be gradually decreased from the upper end to the lower end, but with the present technical level it is hardly possible to solve this problem so that it is impossible in practice to change the cooling effect of a mold.
In order to solve the above and other related problems extensive studies and experiments had been conducted and now the method for making a mold with clad metal plates made by explosive welding has been widely used to increase the thickness of the metal lining, thereby increasing the lifetime of the mold. According to this method, opposed to the conventional metal plating method, the thickness of the metal lining if not limited and the metal lining in any desired thickness may be bonded to the base metal or main body of the mold. In addition, the metal lining or cladding metal sheet may be so machined as to gradually decrease the thickness from the upper end to the lower end so that the performance of the mold may be remarkably improved as compared with the molds lined with the conventional metal plating method.
In general, the conventional methods for making clad metal sheets for molds for continuous casting may be divided into two types. In one type (1) a clad metal sheet is pressed so that a cladding sheet may have a desired form and thereafter the base plate is machined flat, while in the other type (2) a base metal is machined or otherwise to have a desired surface configuration and a cladding metal sheet is bent to match with the surface of the base metal. Thereafter they are spaced apart from each other by a suitable distance and an explosive is ignited to bond them.
Either of the above methods (1) and (2) is selected depending upon the dimensions, configurations and objects of molds, but they have inherent defects to be described below.
First the method (1) has been used for welding flat sheets face-to-face so that the bond is extremely strong and highly reliable. However, the facing or cladding metal which becomes the lining surface of a mold is subjected to press forming so that the dimensional accuracy is by far inferior to that attained by the machining. As a result, an additional machining is required to attain the accuracy that a mold requires. In addition, a cladding metal sheet must have a peripheral margin equal to or twice as much as the thickness (at least more than 50 mm when the thickness is less then 50 mm) so that it may be firmly clamped in position during the pressing. As a consequence the material loss is increased. In addition, a die must be provided to form a desired curved surface for some molds.
According to the second method (2) the cladding metal is pressed to form a surface matching with that of the base metal and the base and cladding metals are spaced apart from each other by a suitable distance when they are explosive welded. However it is difficult to form the surface of the cladding metal with a high degree of accuracy so that the bond between the base and cladding metals is not reliable and separation occurs very often. In addition, the explosive welding often causes the deformation of the curved surface of the base metal so that the deformed surface must be corrected. A further serious defect of the second method (2) arises from the fact that the curved surfaces of the base and cladding metals are bonded together by the explosive welding. As a result, as compared with the cladding between the flat surface base and cladding metals the bond is not as reliable even though they are dimensioned with a higher degree of accuracy.